This invention relates to heterostructure injection lasers and improvements relating to their structure, operation and yield.
One of the most successful semiconductor lasers to date are the channel substrate lasers which may be planar, such as, disclosed in U.S. Pat. No. 4,099,999 or may be nonplanar, such as, disclosed in U.S. Pat. No. 4,033,796. The channel substrate planar (CSP) laser has recently enjoyed some commercial success, offering stable single transverse and longitudinal mode operation. However, the fabrication of these lasers has not been easily iterative causing very low yields from a single wafer and, as a consequence, high production costs and high pricing in the market place.
The major problem in reproduction of the CSP laser is two fold. First, it is difficult to closely control the thickness of the initially grown layer on the substrate of the device so that the layer is of uniform thickness in the layer region on adjacent sides of the substrate channel. For optimal design, these regions should be thin in thickness, such as, 0.2-0.5 .mu.m. Such thicknesses are difficult to obtain and systematically reproduce, particularly in liquid phase epitaxy processing. These thicknesses may be exceeded while growth of the initial layer on the substrate is progressing in epitaxially filling the substrate channel to the desired dimension.
Secondly, it is difficult to obtain uniform growth of the initial layer because in the process of epitaxally filling the substrate channel with the initial layer, the growth rate on either side of the channel is usually different from one another. The principal reason for this difference in growth rates is that most substrate wafers are off in crystal orientation. As a result, layer thickness variations and layer interface ripples and irregularities occur during crystal growth. These irregularities effect the efficient operation of the laser by causing light scattering, increasing current thresholds, and producing off axial, far field beam patterns due to tilted optical phase fronts.
Another problem with conventional CSP lasers is that the emitting aperture of the laser tends to be asymmetric. For beam focusing applications, this may represent a cost disadvantage, as a cylindrical lens will be required for effective focusing. Such lenses are much more expensive than spherical lenses. Spherical lenses could be employed if the beam is substantially symmetrical.